1. Field of the Invention
The present invention relates to a reader-writer terminal device for a contactless IC card (smart card), a radio frequency identification (RFID) card or a contactless data carrier (hereinafter these are collectively referred to as a contactless IC card), a communication system including the reader-writer terminal device and the IC card, and a contactless data carrier incorporating the IC card and the reader-writer terminal device.
2. Description of the Related Art
In data communication using an IC card, a command response system in which one is a command transmitting side and the other is a response transmitting side in receiving a command is generally adopted. However, there is a case in which the receiving side and the transmitting side are not fixed at the initial point of time of the data communication, or there is a case in which the receiving side and the transmitting side replace each other in the middle of the data communication. Even in these cases, the transmitting side and the receiving side are fixed by dividing the data communication into short periods.
In data communication using an IC card, the time interval from the receipt of a command to the output of a response is usually defined by standard, individual specifications, etc. The time interval from the completion of the transmission of the command to the start of the receipt of the response is referred to as a waiting time. Data communication with a shorter waiting time can make the overhead time short and have an advantage in communication performance.
On the other hand, the IC card side in the contactless data carrier usually does not have a battery and a clock signal generating circuit. In this case, the reader-writer terminal side must maintain transmission of a carrier to supply power to the IC card side or reproduce a system clock signal.
Further, on the IC card side, a load modulation scheme is used as a response transmitting means. The switching system switches the load of the IC card on the IC card side, that is, varies carrier amplitude by increasing or reducing a load of an antenna on the reader-writer terminal side transmitting the carrier in accordance with variance of the impedance at an antenna end on the IC card side. According to such system, the response is received on the reader-writer terminal side by detecting the variation in carrier amplitude at the antenna end on the reader-writer terminal side.
A transmission circuit and a receiving circuit are mutually connected via the antenna on the reader-writer terminal side for transmitting a command to the IC card side and receiving the response. Thus, during the transmission of the command, a transmission signal from the transmission circuit creeps into the receiving circuit by itself. If the difference between the large amplitude of an input signal creeping into the receiving circuit during the transmission of the command and the small amplitude of a response receiving input signal is large, an amplification circuit in the receiving circuit is saturated by the large amplitude input during the transmission of the command. The receiving circuit cannot receive the response until the saturated state disappears.
FIG. 1 is a block diagram showing a schematic structure of the transmission circuit and the receiving circuit in a conventional reader-writer terminal device. Both a transmission circuit 22 and a receiving circuit 23 are connected to an antenna 21. The receiving circuit 23 composed of a detecting circuit 23a, a low pass filter (LPF) circuit 23b, an amplifying circuit 23e and a digitization circuit 23d. 
FIGS. 2A-2E illustrate examples of signal waveforms at main portions in FIG. 1, respectively. FIG. 2A illustrates an antenna waveform, FIG. 2B illustrates an output waveform (a detection waveform) of the detecting circuit 23a, FIG. 2C illustrates an input waveform of the amplifying circuit 23e, and FIG. 2D illustrates an output waveform of the digitization circuit 23d. 
As shown in FIG. 2A, a carrier signal is modulated by the command (transmission data) at the transmission circuit 22 during a command transmission period and transmitted from the antenna 21. FIG. 2A illustrates an example in which the carrier signal is modulated with 100% modulation, when the data is transferred between the reader-writer terminal device and the IC card. The transmission of the response is started from the IC card after the lapse of the waiting time just after the transmission period.
Since the large amplitude signal creeps into the receiving circuit 23 via the antenna 21 during the command transmission period, the detecting circuit 23a, LPF circuit 23b, amplifying circuit 23e and digitization circuit 23d operate respectively, digital data corresponding to the command is generated as shown in FIG. 2E. Since the large amplitude signal is input into the amplifying circuit 23e, it operates in a saturated state during the waiting period just after the command transmission period. As a result, the receiving circuit 23 enters a response receiving period, and when receiving the response from the IC card, the input waveform into the digitization circuit 23d hardly reaches a threshold voltage (VREF), then the response at an initial response in the response receiving period becomes impossible to be received. The period in which the response cannot be received becomes a bottleneck, it becomes impossible to shorten the waiting time then performance of the whole of the communication system is prevented from being improved.
Such problems can occur not only between the reader-writer terminal device and the IC card but also in the contactless data carrier equipped with both a reader-writer terminal part having the function of the reader-writer terminal device and a contactless IC card part having the function of the contactless IC card. As examples of the contactless data carrier, a cellular phone, a personal digital assistant (PDA), etc., incorporating the IC card part having a function of short-distance data communication, respectively, are cited.
In Jpn. Pat. Appln. KOKAI Publication No. 11-46354, a sound/image transmission device is described, wherein the transmission device multiplexes and transmits sound and an image, and separates them into the sound and the image to reproduce them on the receiving side, in which the operation of the receiving part is stopped during transmission of image blocks, and the transmission state is switched to the receiving state when the transmission of the image blocks is completed.